The present invention relates to a cryogenic distillation unit, in particular for distilling air, of the type comprising, on the one hand, distillation equipment including at least one cryogenic distillation column and at least one heat exchanger and, on the other hand, means for thermally insulating this equipment. It applies, for example to oxygen-production units on board offshore oil platforms or barges.
U.S. Pat. No. 4,038,060 discloses a cryogenic distillation apparatus for a radioactive gas which prevents dangerous leakage by surrounding the apparatus with a double wall filled with perlite and utilizing an adsorbent within the inner wall for adsorbing radioactive gases in case of an accident.
In conventional technology, cryogenic distillation columns and all the associated equipment (heat exchangers, cryogenic pumps, cryogenic valves, connecting pipes, etc.) operating at low temperature are arranged in a xe2x80x9ccold boxxe2x80x9d, generally of parallelepipedal shape, filled with an insulator in loose bulk form such as expanded perlite. This insulator thermally protects each component both from the external temperature and from that of other components which are at a different temperature. This type of material derives its insulating properties from both low thermal conductivity ( less than 0.05 W/m.xc2x0 C.) and a high head loss, which is favourable in terms of the convection phenomenon.
However, this technology has a drawback from the point of view of safety. By way of explanation, if there is a significant leak of gas or cryogenic liquid, for example due to an accidental pipeline break, the large head loss of the perlite hinders discharge of this fluid to the atmosphere. This results in abrupt cooling of the wall of the cold box and the structure supporting it, which may lead to breaks due to embrittlement and spillage of the cryogenic fluid to the surroundings, which is dangerous to the personnel.
Offshore oil platforms produce residual gases. For economic and environmental reasons, it is becoming increasingly necessary to recover these gases. One method consists in converting them into heavier hydrocarbons, which are in liquid form and are therefore easier to transport, using the Fischer-Tropsch process, which consumes large amounts of oxygen.
It would therefore be beneficial to be able to install an air distillation column on board a platform or a barge, but the safety problem explained above has to be solved because the distillation unit is then fixed on a metal structure and, furthermore, the personnel cannot evacuate quickly from the surroundings of this unit.
The object of the invention is therefore to provide a cryogenic distillation unit comprising effective protection against leaks of cryogenic liquid.
To this end, the invention relates to a cryogenic distillation unit according to claim 1.
The distillation unit according to the invention may comprise one or more of the following features:
the solid insulator located around and above the distillation equipment is an insulator in loose bulk form, in particular expanded perlite;
a solid insulator supporting the inner wall of the enclosure is interposed between the floors of the two walls;
the distillation column and the heat exchanger are supported by legs which pass with a leaktight seal through the floor of the inner wall and bear on that of the outer wall;
at least one shaft is provided for access to auxiliaries of the distillation equipment, such as pumps or valves, this shaft passing through the double wall and being closed off by a removable plug;
the heat exchanger, and optionally other internals of the enclosure whose operating temperature is different from that of the distillation column are provided with additional insulation, in particular by lagging with an insulator in sheet or strip form;
the inner wall is equipped with a valve for limiting its internal pressure;
the enclosure is provided with means for introducing a dry gas, in particular an inert gas, inside the inner wall and/or inside the wall cavity;
the wall cavity is equipped with a valve for limiting its internal pressure;
the enclosure is fixed to an offshore structure such as a oil platform or a barge; and
the enclosure is fixed to a support which is itself fixed to a base of the said structure by means of piles.